The dosing of liquid synthetic-resin components, especially polyols and isocyanates for the production of polyurethane, must be carried out with the utmost care and concern for optimum mixing and displacement parameters if the desired results are to be obtained.
For precise dosing, numerous processes and devices have been proposed. For example, accurate dosing of the components to the mixing head can be effected for example by means of high speed axial-piston pumps.
However, if the synthetic-resin components have abrasive characteristics, i.e. contain abrasive fillers which operate physically and not chemically upon the moving parts, rapidly operating piston pumps are not suitable because they suffer significant mechanical wear.
Synthetic-resin components are considered abrasive when they are highly filled, i.e. when they contain various inorganic and organic fillers, extenders or pigments such as barite, glass fiber and clay. In the production of polyurethanes, such additives are commonly supplied to the polyol component.
It is nevertheless possible to operate with synthetic-resin components containing abrasive constituents when the displacement is effected by slowly operating piston pumps adapted to perform a single stroke for each dosing operation, i.e. to displace the entire quantity of the component required for a single molding operation to the mixing head in a single stroke. Such slowly operating piston pumps are known.
In German open application (Offenlegungsschrift) DT-OS 1454898, there is described an apparatus for the dosing of synthetic-resin components whereby the change in the displacement quantity is effected by the substitution of piston pumps having pistons of different diameters and by a stepless variation of the stroke of these pumps. This arrangement has the disadvantage that it requires a large supply facility, substantial mounting work and high capital expenditure since a common support for the various pumps is expensive.
It is also known to carry out the dosing with slowly operating piston pumps which have effective piston surfaces which can be varied in steps. This conventional arrangement is expensive to construct originally and has the operating disadvantage that the piston surfaces are stepped and hence a stepless control of the operation of the device is not possible.
In the dosing of abrasive-filled synthetic-resin materials using dosing or metering pistons, the metering piston generally is provided with seals which cooperate with the cylinder wall for displacing the synthetic-resin material from this cylinder. When the fillers for the flowable synthetic-resin component or components are highly abrasive, the seal or packing of this piston is subjected to especially heavy wear and, from time-to-time, the cylinder assembly must be rendered inoperative and taken apart to replace or adjust the packing so as to restore effective sealing. This is obviously a time-consuming, expensive and inconvenient process which gives rise to considerable interruption of the operations of the molding system.